So-called "ribbon" cables are presently popular and in general use for conducting a plurality of electrical signals. Ribbon cables usually comprise a large number of cylindrical stranded or solid conductors extending in parallel and spaced equidistant from each other in a single plane. These conductors are covered by a polymeric insulation in the shape of cylinders surrounding each conductor, with each cylinder of insulation being joined to the adjacent cylinder between adjacent conductors to produce profiled major surfaces on each side of the cable defined by arcuate ridges aligned with each conductor and grooves bisecting the distance between conductors.
These cables are commonly used in conjunction with connectors having U-shaped contacts designed to cut through the insulation layer and contact the underlying conductor. The profiled shape of the major surfaces of the cable allows the cable to be conveniently and accurately aligned with the connector contacts prior to "mass termination" of the connection between the cable and the connector which is accomplished by forcing the cable into the U-shaped contacts with a press and thus completing all conductor connections at once.
A drawback of these mass-terminable ribbon cables, particularly when used in a shielded format, is that the insulation layer must necessarily be fairly thin to allow for profiling of the insulation and driving of the contact through the insulation. Thin insulation results in a cable which is not suitable for high impedance, very high quality transmission of signals. It is well known to increase the impedance of the cable, and thus its ability to transmit signals without introducing distortion, by increasing the thickness of the insulation surrounding the conductors. Attempts have been made to retain the advantageous benefits of conventional shielded ribbon cable while increasing impedance by covering both major surfaces of the ribbon cable with layers of insulation, as shown in FIGS. 1 and 2. These layers can be either loose (FIG. 1) and retained by a metal shield wrapped around the layers or bonded to the ribbon cable (FIG. 2) by such means as an adhesive.
Although these methods increase impedance, the first method allows the distance of the shields to the conductors to vary which results in impedance variation, increased crosstalk and variation in signal propagation velocity. In method two, the addition of adhesive is unsatisfactory because the adhesive is typically of a higher dielectric constant and loss tangent than the primary insulation, causing increased signal loss and lower propagation velocities. In addition, the adhesive does not remove cleanly with the dielectric spacer when the cable is prepared for termination. Permanent attachment of the added insulation to the ribbon cable also makes mass termination of the ribbon cable difficult as a result of the total cable thickness being too large for insulation displacement connectors.